1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst.
2. Description of the Related Art
Many automotive vehicles such as automobiles are equipped with a three-way catalyst as an exhaust gas-purifying catalyst. The three-way catalyst contains precious metals as catalytic metals. The precious metals promote the oxidation reactions of hydrocarbons (HC) and carbon monoxide (CO) and the reductive reactions of nitrogen oxides (NOx).
However, the precious metals, in particular, palladium and platinum are prone to suffer from poisoning by HC. When such poisoning occurs, the reduction of NOx becomes hard to occur.
Jpn. Pat. Appln. KOKAI Publication No. 11-207183 describes an exhaust gas-purifying catalyst that contains a refractory inorganic oxide, a cerium-based composite oxide supporting palladium, and a sulfate suppressing the poisoning of palladium by HC. As the refractory inorganic oxide, described are zirconia, alumina, silica, titania and magnesia. As the sulfate, described are sulfates of barium, calcium, strontium, cesium, potassium, magnesium, yttrium and lanthanum. This exhaust gas-purifying catalyst is obtained by immersing a monolith substrate into slurry containing the refractory inorganic oxide, the cerium-based composite oxide supporting palladium, and the sulfate, and then drying the monolith substrate drawn up from the slurry.